Hello Reinout,
Yes, I've wondered about the choices myself.
I would imagine a few were chosen because of the amount of information available on their victories. There are detailed letters from von Tutschek and
Erwin Böhme that have been translated into English and that provide a good deal of information on some of their victories. The more you know about a particular victory from the German side, the easier it is to narrow down the possible identity of the opponents. Many of Göring's combat reports and flight log entries have also survived and have been translated, as well. I'm sure name recognition had something to do with it also.
You're right about less information being available about French opponents, I'm sure. Also, some of the top aces like Udet, Berthold, and Löwenhardt scored the majority of their victories rather late in the war; with the increased scope and pace of the aerial war that occurred in the Allied offensives of late 1918, it gets more and more difficult to pick out exactly who so-and-so shot down in those hectic, large-scale combats. This is the case with some of Lothar von R's final combats, as evidenced in the book.
Just my thoughts.
Greg